The present invention relates to contact lenses, and more particularly, to clinically-acceptable silicone-based soft gas permeable contact lenses and methods for their manufacture.
The development and use of plastic materials and compositions for contact lenses has been the subject of much attention over the years.
Among the first such developments was the so-called hard lens utilizing the hard plastic polymethylmethacrylate (PMMA). However, this material does not exhibit a significant degree of oxygen permeability and has very poor surface wettability characteristics. The art then progressed to softer lenses based upon poly 2-hydroxethylmethacrylate (poly HEMA), a material having significantly better oxygen permeability and surface quality than the hard PMMA plastic. Nevertheless, these characteristics were still not as high as desirable or necessary, and lenses of this type often resulted in serious problems of corneal staining, swelling, ulcers, thickness and infection.
Somewhat more recently, based upon better understanding of the corneal requirement of substantial oxygen permeability, the art proposed the use of so-called hard gas permeable (HGP) lenses composed of either of two types of plastic materials, acrylic silicone or acrylic fluorosilicone. See, e.g., U.S. Pat. No. 3,808,178. Generally, the oxygen permeability of HGP lenses can progressively be increased with increasing amounts of the silicone and/or fluorosilicone in the composition; at the same time, however, the surface wettability of the lens becomes progressively poorer. In order to overcome this problem, it is known to incorporate a relatively large amount of methacrylic acid, an ionic material, into the formulation, resulting in the lens surface being negatively charged. While this expedient does lead to improved surface wettability, the negatively-charged surface has a very high absorptivity leading to serious deposition problems. As a consequence, the HGP lens is of only limited potential.
Most recently, hydrophilic soft gas permeable (SGP) lenses have been developed based upon compositions containing, e.g., a polymerizable vinylic siloxane and a hydrophilic vinylic monomer. See, e.g., U.S. Pat. Nos. 4,136,250; 4,139,513; 4,182,822; 4,261,875; 4,343,927; 4,426,389; 4,486,577; 4,605,712; 4,711,943; and 4,837,289. The SGP lenses of this type have excellent oxygen permeability and hydrophilicity. Surprisingly, however, although the lens is highly hydrophilic, its functional (i.e., on the eye) wettability is still relatively poor, and the lens is highly irritable and uncomfortable, often resulting in serious deposition problems, making it highly unsuitable for extended wear.
In the manufacture of early-generation soft contact lenses, the lenses were subjected to an aqueous or physiological saline solution extraction to render them clinically acceptable. See, e.g., U.S. Pat. No. 4,158,089 (Col. 6, line 25) and U.S. Pat. No. 3,937,680 (Col. 6, line 51). The more recently-developed SGP lenses are also subjected to this conventional treatment, i.e., extraction with physiological saline solution. See, e.g., U.S. Pat. No. 4,139,513 (Col. 9, line 16); U.S. Pat. No. 4,261,875 (Col. 5, line 43) and U.S. Pat. No. 4,711,943 (Col. 37, line 32), involving immersion of the lens in a 50/50 v/v methanol/water mixture. The present invention is predicated on the discovery that this form of treatment does not effectively dissolve and extract the residual siloxane monomers and/or their low molecular weight derivatives formed during polymerization. Thus, silicone-based SGP lenses of the prior art contain a substantial amount of residual siloxane monomer and derivatives, resulting in a high degree of irritability, poor wettability and serious deposition problems.
It should be mentioned that the non-polar solvent hexane has been taught in the pre-treatment of a silicone rubber lens, as illustrated in U.S. Pat. No. 3,350,216. However, it has been found that hexane is not suitable for use in the extraction of silicone-based SGP lenses.
It is the primary object of the present invention to provide silicone-based SGP lenses, and processes for their production, which have enhanced clinical acceptability by reason of greatly reduced levels of residual monomers or other reactants and/or low molecular weight by-products of the polymerization reaction used to produce the lens material.